You missed the obvious fact that the bug change might happen to an API you already use. It doesn't have to be a new API -- just a new release of it by Apple.
So the problem remains: you can't easily extend the class with your own fix to bring out a bugfix release for your app.
Apple writes a new API. It has a problem. You try to adopt it, it doesn't work. You don't just ship that, you do something else.
Unpleasant, but doesn't hit the user.